On and after; from a certain time onward (including that time); commonly used in written/formal contexts.
6月1日以降は新しい料金が適用されます。
From June 1 onward, the new rates will apply.
午後5時以降は入場できませんので注意してください。
Please note that entry is not allowed after 5 PM.
この日以降、彼とは連絡を取っていない。
I haven't contacted him since that day.